Why not reword to avoid confusion?

> ...within the callback, the length of <code>char</code> should always be
> 1.

and, as you suggested, 

> ...<code>char</code> instances are defined in multiple scopes...


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EDIT: Added some style guide references to support my answer. 

[Yahoo style manual][1] recommends re-wording, as does [reference.com][2], 
and [The manual of scientific style: a guide for authors, editors, and researchers
 By Harold Rabinowitz][3] section [3.6.7.ii-iii][4]

On the other hand, the [Chicago Manual of Style says][5]

> The plural of a word or phrase in quotation marks is now formed
> without anapostrophe—that is, with the addition of s or es within the
> quotation marks. Note the example: “To be continueds”

Although it does not address possessives of items in quotes.

  [1]: http://styleguide.yahoo.com/editing/punctuate-proficiently/quotation-marks
  [2]: http://dictionary.reference.com/help/faq/language/s30.html
  [3]: http://www.sciencedirect.com/science/book/9780123739803
  [4]: http://books.google.com/books?id=YZCqJzffR20C&pg=PA177&lpg=PA177&dq=The%20manual%20of%20scientific%20style:%20a%20guide%20for%20authors,%20editors,%20and%20researchers%20By%20Harold%20Rabinowitz%20plural%20quotation%20marks&source=bl&ots=eGnPM7cFfm&sig=GvI0HBAFwgkPJmVYammKEBk5o2o&hl=en&sa=X&ei=_WEDT4j-GcvpgQeqmJmVAg&ved=0CC0Q6AEwAg#v=onepage&q&f=false
  [5]: http://www.chicagomanualofstyle.org/about16_rules.html